This reseach is aimed at uncovering the cellular basis of behavioral hierarchies (choice) and associative learning (classical conditioning, taste aversion learning) at the level of the single neuron. Experiments are performed on a relatively simple organism, the gastropod mollusk Pleurobranchaea. Our research has six broad aims. First we wish to understand how feeding behavior maintains its dominant position in the behavioral hierarchy. We will test the hypothesis that the central motor network that controls feeding also inhibits the withdrawal behavior. Second, we shall study how egg laying behavior exerts its hormonal suppression of feeding behavior. Toward this end we shall apply the purified polypeptide hormone that causes egg laying to the nervous system during neurophysiological analysis of the feeding network. Third, we shall continue to analyze a demonstrated learning analog in the isolated nervous system, using a paradigm that is patterned after classical conditioning of the feeding behavior of intact specimens. Fourth, we shall perform behavioral experiments on taste aversion learning. Sixth, since both behavioral hierarchies and associative learning involve the feeding behavior, we shall perform a detailed neurophysiological analysis of the central motor network that controls feeding behavior. The research has health-related significance to learning mechanisms and disabilities, and to polypeptide hormone disorders such as diabetes.